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- exceptional conditions during expression evaluation are undefined
- "nil" for interfaces, comparison against "nil" R=r DELTA=38 (24 added, 5 deleted, 9 changed) OCL=16207 CL=16211
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@ -47,11 +47,9 @@ Open issues according to gri:
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(issue: what happens in len() + const - what is the type?)
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[ ] Do composite literals create a new literal each time (gri thinks yes)
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[ ] consider syntactic notation for composite literals to make them parseable w/o type information
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[ ] nil and interfaces - can we test for nil, what does it mean, etc.
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[ ] type switch or some form of type test needed
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[ ] what is the meaning of typeof()
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[ ] at the moment: type T S; strips any methods of S. It probably shouldn't.
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[ ] talk about underflow/overflow of 2's complement numbers (defined vs not defined).
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[ ] 6g allows: interface { f F } where F is a function type. fine, but then we should
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also allow: func f F {}, where F is a function type.
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[ ] provide composite literal notation to address array indices: []int{ 0: x1, 1: x2, ... }
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@ -60,6 +58,8 @@ Decisions in need of integration into the doc:
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[ ] pair assignment is required to get map, and receive ok.
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Closed issues:
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[x] nil and interfaces - can we test for nil, what does it mean, etc.
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[x] talk about underflow/overflow of 2's complement numbers (defined vs not defined).
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[x] change wording on array composite literals: the types are always fixed arrays
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for array composites
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[x] meaning of nil
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@ -1249,6 +1249,11 @@ TODO(gri) This may be overly constraining. What about "len(a) + c" where
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c is an ideal number? Is len(a) of type int, or of an ideal number? Probably
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should be ideal number, because for fixed arrays, it is a constant.
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If an exceptional condition occurs during the evaluation of an expression
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(that is, if the result is not mathematically defined or not in the range
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of representable values for its type), the behavior is undefined. For
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instance, the behavior of integer under- or overflow is not defined.
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Operands
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----
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@ -1511,9 +1516,6 @@ Operators combine operands into expressions.
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unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
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TODO: If we allow non-blocking sends only in the form "ok = ch <- x", it doesn't
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make sense to give binary "<-" precedence 3. It should be at the lowest level. TBD.
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The operand types in binary operations must be equal, with the following exceptions:
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- The right operand in a shift operation must be
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@ -1625,7 +1627,8 @@ Comparison operators
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Comparison operators yield a boolean result. All comparison operators apply
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to strings and numeric types. The operators "==" and "!=" also apply to
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boolean values and to pointer types (including the value "nil").
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boolean values and to pointer types (including the value "nil"). Finally,
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"==" and "!=" can also be used to compare interface types against "nil".
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== equal
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!= not equal
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@ -1634,7 +1637,12 @@ boolean values and to pointer types (including the value "nil").
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> greater
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>= greater or equal
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TODO: Can we/should we be able to compare interfaces?
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Strings are compared byte-wise (lexically).
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Interfaces can be tested against "nil" (§Interface types).
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For a value "v" of interface type, "v == nil" is true only if the predeclared
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constant "nil" is assigned explicitly to "v" (§Assignments), or "v" has not
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been modified since creation (§Program initialization and execution).
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Logical operators
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@ -1845,11 +1853,22 @@ Expression statements
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IncDec statements
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----
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The "++" and "--" statements increment or decrement their operands
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by the (ideal) constant value 1.
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IncDecStat = Expression ( "++" | "--" ) .
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a[i]++
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The following assignment statements (§Assignments) are semantically
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equivalent:
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Note that ++ and -- are not operators for expressions.
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IncDec statement Assignment
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x++ x += 1
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x-- x -= 1
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Both operators apply to integer and floating point types only.
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Note that increment and decrement are statements, not expressions.
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For instance, "x++" cannot be used as an operand in an expression.
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Assignments
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@ -2522,12 +2541,12 @@ Program initialization and execution
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----
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When memory is allocated to store a value, either through a declaration
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or new(), and no explicit initialization is provided, the memory is
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or "new()", and no explicit initialization is provided, the memory is
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given a default initialization. Each element of such a value is
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set to the ``zero'' for that type: "false" for booleans, "0" for integers,
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"0.0" for floats, '''' for strings, and nil for pointers. This intialization
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is done recursively, so for instance each element of an array of integers will
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be set to 0 if no other value is specified.
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"0.0" for floats, '''' for strings, and "nil" for pointers and interfaces.
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This intialization is done recursively, so for instance each element of an
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array of integers will be set to 0 if no other value is specified.
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These two simple declarations are equivalent:
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